1. Field
This invention relates generally to low voltage detector circuits and more specifically to a low power consumption, low voltage detector circuit disposed on an integrated circuit and fabricated using a complementary metal oxide semiconductor (CMOS) process.
2. Related Art
Electronic devices frequently employ a voltage detector to indicate if a particular voltage is outside a specified range. For example, battery operated devices often include a voltage detector to provide a low voltage indication when the voltage supplied by the battery decreases below a predetermined level. The low voltage indication can notify the user of the device that the battery should be replaced or recharged. In some electronic devices, it is desirable that a low voltage detector (LVD) have a relatively fine resolution, so that the LVD is able to determine when a monitored voltage is within a narrow range. However, such an LVD (hereinafter “high power LVD”) can consume an undesirably large amount of power due to its complexity. A low power consumption, or low power, LVD should have a lowest possible current consumption while also satisfying robustness requirements for its intended application.
A microcontroller unit (MCU) is a single integrated circuit that may include one or more microprocessor cores, and, in some embodiments, random access memory, read-only memory, a clock, an input/output control unit, and one or more LVDs. An LVD may allow an MCU to switch to a known (and safe) state whenever its power supply voltage does not meet a minimum value.
An MCU has at least two operating modes. An MCU has a full power, or run, mode during which time many circuits are operating and, as a result, when in run mode, an MCU may consume maximum power. An MCU may include a high power LVD that operates during run mode. An MCU has a low power, stop, or standby, mode during which time its high power LVD is typically either not operating, or operating intermittently, to save power; however, during which time its low power LVD is typically operating continuously. When either type of LVD detects that the power supply voltage for an MCU is below a minimum value, the LVD sets a flag. Such a flag may force an asynchronous reset of an MCU as a means to prevent the MCU from entering an unpredictable (and consequently undesirable) state, or the flag may generate an interrupt to run a specific subroutine for a low voltage condition. An LVD has a trip point, which is the value of the power supply voltage at which the LVD sets the flag.
In a typical MCU, the trip point of a low power LVD is higher than the trip point of a high power LVD. During either run mode or standby mode, as the power supply voltage drops below the trip point of the low power LVD, the low power LVD sets a flag, generates an interrupt or wakes up the MCU (and may also cause the high power LVD to turn on, if it is not on), and the MCU performs procedures that may be needed prior to powering down, such as by saving information in non-volatile memory. Thereafter, if the power supply voltage drops below the trip point of the high power LVD, then the MCU resets.
High power LVDs usually consume tens to hundreds of microamperes (μA) because they require voltage dividers and bandgap reference voltages to properly operate with high accuracy. When an MCU is powered from a battery, a lower current consumption by a low power LVD results in longer battery life.
Some LVDs disadvantageously use a reference voltage that varies with the threshold voltage (VT) of a metal oxide semiconductor field effect transistor (MOSFET). The threshold voltage of a MOSFET depends upon process and the threshold voltage changes greatly with temperature. When an LVD uses a reference voltage that depends upon threshold voltage, the part-to-part variation of the LVD is disadvantageously great. Therefore, such an LVD is either disadvantageously affected by temperature changes, or it requires additional circuitry that provides temperature compensation but which consumes additional power. Moreover, such an LVD needs trimming to counterbalance the part-to-part variation.
Using the advanced compact model for a MOSFET, the inversion level of a MOSFET transistor is determined by an inversion factor if, which is defined as if=I/Is, where I is the drain current of the transistor, and Is is the normalization current. The normalization current Is is equal to ISQS, where ISQ is the sheet specific current that is defined by certain process parameters and S is the aspect ratio of the transistor. The aspect ratio S of a MOSFET transistor is the ratio of channel width W to channel length L. Furthermore,ISQ=nμC′ox(ΦT2/2)
where μ is the mobility of the carriers in the channel, n is the slope factor, C′ox is the oxide capacitance per unit area of the gate, and ΦT is the thermal voltage.
The thermal voltage ΦT is a function of temperature and increases directly proportionately with increasing temperature. The thermal voltage ΦT=kT/q, where T is the temperature measured in kelvins (abbreviated “K”, and sometimes informally referred to as “degrees Kelvin”), and q is the magnitude of the electrical charge of an electron (1.6022×10−19 coulombs). The Boltzmann's constant, k, can be expressed as 1.3807×10−23 joules per kelvin. The thermal voltage ΦT is approximately 25.85 millivolts at room temperature (approximately 300K). At room temperature, the thermal voltage ΦT changes at a rate of approximately 0.0862 millivolts per degree Celsius.
Weak inversion, moderate inversion and strong inversion describe different operational modes of a MOSFET. Weak inversion occurs when a drain current of a MOSFET transistor is dominated by a diffusion current, moderate inversion is when the drain current has both diffusion current and drift current components, and strong inversion is when the drain current is dominated by the drift current. In a MOSFET, inversion occurs when a thinner channel is formed in the transistor in the substrate region under the gate. When there is no channel, the transistor is at cut-off. As a rule of thumb, a MOSFET that has an inversion factor of less than “1” is said to be in weak inversion; a MOSFET that has an inversion factor of about “1” to “100” is said to be in moderate inversion; and a MOSFET that has an inversion factor of greater than “100” is said to be in strong inversion.